In our technical world displays have an important function as human interfaces for making abstract information available through visualization. In the past, many applications for displays were identified and realized, each with its own specific requirements. Therefore, different display technologies have been developed, each having their own strengths and weaknesses with respect to the requirements of particular display applications, thus making a particular display technology best suited for a particular class of applications.
The most important display applications being pursued are based on cathode ray tubes (CRT), liquid crystal displays (LCD), or vacuum fluorescent, plasma, light emitting diode (LED), electroluminescent, and electromechanic displays. Among the most decisive criteria dictating an appropriate display technology are cost of fabrication, power efficiency, reliability, weight, size of screen, depth, brightness, gray-scale capabilities, dynamic range, resolution (i.e. the minimum size of an addressable picture element on the display), contrast, dependence of the contrast on the viewing angle, switching speed of a particular pixel (pixel element), sunlight readability, color range, chrominance and chrominance contrast. CRTs have a dominant position on the display market due to their low price and their multicolor, high resolution and gray-scale capabilities. However, they have disadvantages if low weight, low power, small depth and sunlight readability is desired, for example in applications for battery driven portable computers.
The other mentioned display technologies come into play in areas where CRTs show weaknesses, especially if the weight, the depth and/or the power consumption of CRTs are simply not acceptable. For example, in applications for wrist watches or portable computers--an important domain of flat panel displays--or, generally, in applications which require large and/or flat displays, alternate technologies are preferred.
Due to their advantages--low weight, low power consumption, low depth--LCDs became the dominant flat panel display technology. The LC-material is cheap and the fabrication processes are scalable although at considerable cost, so that displays of arbitrary size can be made. However, many applications such as high resolution graphics or full motion video require high resolution, often in combination with high pixel switching speed. In these cases, LCD technology has drawbacks. In LCDs, high resolution is achieved with x-y matrix addressing techniques, thus reducing the number of address lines. However, x-y matrix addressing, in combination with the fundamental physical properties of LC materials, lead to high resolution only at the expense of a poor contrast between adjacent pixels, a small maximum viewing angle and other effects further deteriorating image quality, for example cross talk between pixels. These drawbacks can be partly overcome with so called `active` x-y matrix addressing (see "A gray-scale addressing technique for thin-film-transistor/liquid crystal displays" by P. M. Alt et al., IBM Journal of Research and Development Vol. 36, No. 1, pp. 11-22, 1992). However, active matrix addressing requires a network of transistors of the same size as the display itself, each transistor controlling the charge stored in one capacitor, each capacitor influencing the orientation of the LC molecules between its electrodes and thus defining one pixel of the entire display. Today, the active matrix addressing makes possible brilliant full color displays capable of graphics with reasonable resolution or full motion video. However, large flat panel LCDs with active matrix addressing are expensive due to the costs of the fabrication of the transistor matrix array. In spite of the improvements due to the active matrix addressing, undesirable drawbacks remain such as the still limited viewing angle and the resolution being limited today to minimum pixel sizes of 100 .mu.m.times.100 .mu.m. Furthermore, the pixel size determines the maximum magnification factors of projection displays, i.e. displays which generate a secondary virtual or real image from a primary display element by means of optical imaging and thus allow for the generation of magnified images, provided the resolution of the original image prior to projection was sufficient.
Large high resolution LCDs with sizes more than 30 inches diagonal are difficult and expensive to fabricate. Therefore, large flat displays, as desired for high definition TV (HDTV) or public information boards, are the domain of vacuum fluorescent, electroluminescent or plasma displays despite their poorer power efficiencies. In particular, large full-color flat-panel plasma displays have excellent potential to replace cathode ray tubes in HDTV in the near future.
Light emitting diode (LED) displays are flat and lightweight, have sunlight viewability, and have--in comparison with LCDs--an excellent viewing angle and a high response speed of the order of 10-20 ns compared with 10-100 ms for LCDs. In addition, LED displays can have a smaller pixel size than LCD displays. LED displays have a pixel size dictated by the dimensions of a single LED, which can be quite small since it is defined by semiconductor lithography (.congruent.1 .mu.m.times.1 .mu.m or less). The smallest pixel size of LCD displays is (about 100 .mu.m.times.100 .mu.m) dictated by the physical properties (such as size of molecules, viscosity, etc.) of liquid crystals. The rapid modulation speed of LED displays makes it possible to use simple modulation techniques, e.g. on/off switching, for enhancing gray-scale capabilities. For example, for n gray-levels the LED switches on 0-n times inside of a single cycle.
LED displays entered the market place as a replacement for vacuum tube displays and were used as small indicators on instruments and small alphanumeric devices in hand-held computers. However, they lost market share to LCD technology in certain areas where the cost of their substrates and fabrication lead to unfavorable prices, or where the power consumption of LEDs was not competitive. On the other hand, these arguments in favor for LCD technology become less important for several reasons. First, progress in materials science and technology lead to advances in LED devices in terms of their power efficiency and further progress can be expected. Furthermore, the power advantage of LCDs is significant mainly in applications in which the LCD is used in a nonemissive mode, i.e. the LCD is illuminated by ambient light and acts only as reflective or transmissive spatial filter. In this case, power is only used for addressing and switching individual pixels. These are low power processes compared with driving emissive devices such as LEDs. However, there are many applications where LCDs cannot be used in a nonemissive mode, but must be illuminated with lamps, for example in high-brightness displays for laptop computers. In these cases, the power advantage of LCDs becomes questionable. The price argument in favor for LCDs becomes less important, when active matrix x-y addressing must be used. Then, the major part of costs must be attributed to the fabrication of the transistor matrix for addressing.
Therefore, areas of application for LED displays arise where advantageous features such as maximum viewing angle, fast response speed, and minimal pixel size are desired. However, the prospects of LED displays depend on the solutions of numerous problems, for example the color capabilities, the integration of different addressable LED-based light sources of different color on a monolithic chip, and the maximum size of such a monolithic chip.
Whereas 10 inches diagonal LCDs are common even with active matrix addressing and systems twice as large are in an experimental stage, the size of monolithic LED devices is typically between 2 and 25 mm and limited by the available size of the substrates on which the LEDs are fabricated, with the LEDs being normally realized in epitaxial semiconductor layers grown on said substrates. Larger LED based displays have been developed, but not in a monolithic version. They have been assembled using single diodes or monolithic modules having a small number of display points. A display with a resolution of 64 lines per inch, 4 inches by 4 inches in size, with 49,000 light emitting junctions composed of 1/4 inch times 1/4 inch GaP LED arrays, each array containing 16.times.16 light emitting junctions, is known from "Multi-Mode Matrix Flat Panel LED Varactor Concept Demonstrator Display" by K. T. Burnette, Proceedings of the Society for Information Display, Vol. 21/2, pp. 113-126, 1980. Other examples for hybrid LED displays are given in "A High-Brightness GaP Green LED Flat-Panel Device for Character and TV Display" by Tatsuhiko Niina et al., IEEE Transactions on Electron Devices, Vol. ED-26, No. 8, pp. 1182-1186, 1979.
Presently, full (in particular 3) color gray-scale capable matrix-addressable LCDs with 10 inches diagonal and a pixel size of 100 .mu.m.times.100 .mu.m are state of the art. Color capability is achieved by providing a flat display illuminated by white light through a 3-color filter array such that each pixel, the smallest picture element of the display, is subdivided into 3 adjacent independent subpixels, each subpixel representing one of three elementary colors--blue, green and red--which can be mixed with variable intensities such that any single pixel gives the impression of radiating any predefined color of the white light spectrum at any gray-level between zero intensity (defining "black" color) and a predefined maximum value.
So far, multicolor LED flat panel displays have been assembled from discrete LEDs (see for example "A multicolor GaP LED flat panel display device" by T. Nina et al., 1981 SID Int. Symp. Digest Tech. Papers 12, pp. 140-141, 1981). In order to simplify manufacturing of such a device by taking advantage of modern semiconductor integration and processing technology, monolithic multicolor LED based devices providing a multitude of LEDs would be desired, whereby either individual LEDs generate discrete colors tunable over a certain range, or at least two different groups of LEDs exist, each being characterized by a common wavelength of its LEDs, the common wavelengths of different groups being distinguishable. The meaning of the word `distinguishable` in this context depends on the application, namely on the observer. If the observer is a person, the person should be able to distinguish different colors. If the display is interpreted by color-sensitive instruments, the instrument's color sensitivity is relevant.
The above-mentioned list of characteristic features of monolithic devices with integrated multicolor LEDs is not complete, if the display has to show arbitrary image data including chromatic contrasts. Then, monolithic devices providing a two-dimensional array of equivalent multicolor pixels, the pixels being distributed on a flat substrate, would be desired, whereby each pixel is either represented by at least one LED generating a color tunable within a certain range under electronic control and/or is represented by a group of spatially separated LEDs, each being capable of generating one of two or even more distinguishable wavelengths. For a natural representation of any arbitrary visible color, the generation of three wavelengths at the position of each pixel is desired, for example one wavelength belonging to the blue, another to the green and the third to the red part of the visible spectrum.
However, monolithic multicolor LED arrays providing a 1- or 2-dimensional distribution of individually addressable light sources on a substrate and providing 2 or more distinguishable colors are not known in the art. A special unsolved problem is the integration of LEDs capable of covering the entire spectrum of visible light.
In the following, the color capabilities of visible LEDs with special emphasis on the integration of different colors on the same substrate are briefly summarized, as it is known in the art. Semiconductor technologies are known to fabricate single monochrome LEDs for the entire spectrum of visible light. An overview about such technologies is given in textbooks such as "Flat-Panel Displays and CRTs", edited by L. E. Tannas, Van Nostrand Reinhold Company, Chapter 9, pp. 289-331, 1985. In the majority of applications, either direct electronic band-to-band transitions or impurity-induced indirect band-to-band transitions in the material forming the active region of the LED are used for light generation. In these cases, the energy gap of the material chosen for the active region of the LED, i.e. the zone where the electronic transitions responsible for the generation of light within the LED take place, determines the color of a particular LED. A further known concept for tailoring the energy of the dominant optical transition of a particular material and thus the wavelength of the generated light is the incorporation of impurities leading to the introduction of deep traps within the energy gap. In this case, the dominant optical transition may take place between a band-state of the host material and the energy level of the deep trap. Therefore, the proper choice of an impurity may lead to optical radiation with photon energies below the energy gap of the host semiconductor. In this case, the impurities, the host-semiconductor and the exact alloy composition chosen for the active layer offer three degrees of freedom for the design of a LED with a particular wavelength since the bandgap induced shift in the impurity levels in alloys would change the emission color.
Today, exploiting these two concepts for tailoring the emission wavelength of an LED and using III-V or II-VI compound semiconductors or their alloys for the active region of the LED, it is possible to cover the optical spectrum between near infrared and blue with discrete emission lines. However, due to constraints on the growth of high quality semiconductor layers, the general problem arises whether it is feasible to combine materials, doping conditions and device concepts for LEDs such that different wavelengths can be generated from a monolithic LED array.
In the majority of LED technologies, the active region is placed between appropriate semiconductor cladding layers, one being doped p-type and the other being doped n-type, and the optical transitions are induced by injecting electrons and holes into the active layer by applying an appropriate bias between the cladding layers. An important and sometimes restrictive premise of this approach is the existence of proper cladding materials which can be doped p- as well as n-type and can serve as substrates for the fabrication of high quality active regions. Examples of common materials for active regions in p-n-type LEDs and the spectral regions they are best suited for are summarized in the following, whereby spectral data are in general room temperature values. Materials mostly used are III-V semiconductors such as GaAs, GaAlAs, GaP, GaAsP, GaInP, AlGaInP GaN, AlGaN, InAlGaN, and II-VI compounds such as ZnSe/CdZnSe, CdZnSeS or MgCdZnSeS, and the IV--IV compound SiC.
Direct band-to-band transitions in GaAs are used for the generation of infrared light at around 870 nm. Exploiting direct band-to-band transitions in Ga.sub.x Al.sub.1-x As, the infrared/red spectral range between about 867 nm and about 652 nm can be covered by choosing an appropriate molar traction x. The material system GaAs.sub.1-x P.sub.x is suitable for the spectral range 867 nm-610 nm (i.e. infrared--red) when exploiting direct transitions (x&lt;0.49), and appropriate for 610 nm-548 nm (i.e. red--green) when taking advantage of indirect band-to-band transitions which can be enabled by impurity induced processes by doping with isoelectronic impurities such as nitrogen.
For blue light generating LEDs, wide bandgap semiconductors such as SiC, GaN, AlGaN, InAlGaN, ZnSe/CdZnSe or CdZnSeS are candidates. Until recently, the majority of such wide bandgap materials could not be grown p- as well as n-doped. Therefore, LEDs based on the conventional concept of exploiting p-n-junctions for carrier injection into the active region were not feasible. To circumvent this inconvenience, MIS-type diodes (i.e. metal-insulator-semiconductor diodes) have been successfully applied. In MIS-type LEDs, the active layer is made insulating and sandwiched between a conductive semiconductor layer and a metal contact. By applying an appropriate bias V between metal and conductive semiconductor layer, electrons are injected into the active layer, whereby the electron emitter is either the negatively biased metal layer or, if the semiconductor layer is n-doped, the negatively biased semiconductor layer. In the active layer, the injected electrons radiatively recombine with holes, which are refreshed by the counter electrode, the counter electrode being either the positively biased metal layer, if the electron emitter is an n-doped semiconductor layer, or an n- or p-doped semiconductor layer. Such structures show typical diode-like nonlinear current-voltage characteristics including a threshold voltage and an exponential increase of the injected current as a function of the applied bias V. The highest power efficiencies are usually achieved by emitting electrons from a n-type semiconductor layer towards the metal electrode, which serves as anode of the device. The overall performance of a MIS-LED, in particular the relationship between the injected current I and the applied bias V and the relationship between the intensity of the generated light and the injected current I, depend on many physical processes related to the carrier injection (e. g. tunneling, thermal excitation over barriers) and the carrier transport in the active layer (e.g. field ionization of deep impurities, impact ionization of deep impurities, hopping transport of holes, space charge current limitations, etc.). A more detailed discussion of these physical processes is not relevant for the understanding of this application, since an optimization of the performance of MIS-LEDs is not an object of this invention. In this context, it is sufficient to mention the relevance of the thickness of the insulating layer for the power efficiency of MIS-LEDs. For the thickness of the insulating layer, a trade-off exists. If it becomes too thin, an increasing part of the injected electrons passes through the insulating layer directly into the anode of the device without radiative recombination, thus lowering the power efficiency. If the insulating layer is too thick, the series resistance and the threshold voltage increase, again lowering the power efficiency. Typical values for an optimized thickness of the insulating layer, taken for a GaN-based MIS-LED, are in the range 20 nm-1 .mu.m (see "GaN electroluminescent devices: preparation and studies" by G. Jacob et al., Journal of Luminescence Vol. 17, pp. 263-282, 1978).
Blue light emitting MIS diodes have been realized in the GaN system. Examples of these have been published in:
"Violet luminescence of Mg-doped GaN" by H. P. Maruska et al., Applied Physics Letters, Vol. 22, No. 6, pp. 303-305, 1973, PA0 "Blue-Green Numeric Display Using Electroluminescent GaN" by J. I. Pankove, RCA Review, Vol. 34, pp. 336-343, 1973, PA0 "Electric properties of GaN: Zn MIS-type light emitting diode" by M. R. H. Khan et al., Physica B 185, pp. 480-484, 1993, PA0 "GaN electroluminescent devices: preparation and studies" by G. Jacob et al., Journal of Luminescence, Vol. 17, pp. 263-282, 1978, PA0 EP-0-579 897 A1: "Light-emitting device of gallium nitride compound semiconductor".
In these studies, a common substrate for GaN is used, namely sapphire. On the sapphire substrate, a thick (several 100 .mu.m) layer of n-type GaN was grown, often unintentionally doped GaN. On top of the n-GaN layer, the active layer of insulating GaN was grown. The insulating nature was realized by the incorporation of acceptors such as Zn, Cd or Mg during growth which compensate intrinsic donors and thus reduce the conductivity. Metals such as In, Ni, Ag, or Al served as metal contacts to the insulating active layer. As the sapphire substrate is insulating, special attempts are necessary to apply a bias to the MIS-diode. For making a contact to the n-GaN layer, either side contacts at the edges of the substrate are formed, or the n-GaN layer is made accessible from above by etching contact holes through the insulating GaN active layer.
It has also been recognized in the above-mentioned articles that the compensation of the insulating GaN layer by impurities such as Zn, Cd, or Mg can lead to different coexistent impurity levels within the energy gap of the host semiconductor whereby the density of the impurity states depends on the doping conditions, i.e. on the type of impurity, its concentration and/or the growth conditions. It is further known that the dominant electronic transitions which contribute to the electroluminescence of the compensated GaN layer take place between the lowest conduction band and an impurity state within the energy gap. Therefore, depending on the energy of the impurity states involved in the electroluminenscent processes, light is generated with photon energies of the bandgap reduced by the binding energy of the impurity state. Therefore, by appropriate tailoring of the distribution of impurity states, the peak of the GaN electroluminescence spectrum, which is in the ultraviolet if band-to-band transitions are dominant, is red-shifted due to the introduction of impurities. Based on this concept, GaN MIS-LEDs have been fabricated with peak wavelengths in the blue, green, yellow, orange and red part of the spectrum, together spanning the entire visible spectrum. The quantum efficiency as well as the threshold voltage of such devices are related to the color of their radiation. Quantum efficiencies of about 0.5% and 0.1% have been demonstrated for the green-yellow and for the blue part of the visible spectrum, respectively. Typical threshold voltages are 4V for the blue, 5V for the green, and 10V for the yellow.
Recently, due to progress in the development of techniques for p-doping of GaN and related compounds such as InGaN and AlGaN, the first p-n-type blue GaN based LEDs have been demonstrated. One example representing the state of the art is given in "Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes" by S. Nakamura et al., Applied Physics Letters, Vol. 64, No. 13, pp. 1687-1689, 1994. The vertical layer structure of the LED disclosed in this article consists of a stack of GaN/AlGaN/InGaN layers grown on sapphire. The active layer consists of Zn doped InGaN sandwiched between p- and n-doped AlGaN layers, the sandwich forming a double-heterostructure. The Zn doping leads to optical transitions whose energy is related to the energy of Zn-related impurity states in a similar way as it is known for GaN (see above). Since the sapphire substrate of this device is not conductive, contact holes are etched through the active layer in order to get access to the n-GaN layer underneath.
Recent progress in developing doping techniques for II-VI semiconductors such as ZnSe, CdZnSe or CdSSe allows these materials to be exploited for the fabrication of p-n-junction based blue-light-emitting LEDs and even laser diodes. An impression about the state of the art of the growth of II-VI wide-bandgap materials can be taken from the article "Blue-green diode lasers" by G. F. Neumark et al., Physics Today 6, pp. 26-32, 1994.
In summary, using different semiconductor materials, their alloys and the incorporation of impurities, different materials for active layers of LEDs are available to fabricate single LEDs emitting light at wavelengths spanning the entire visible spectrum. However, concepts to integrate different LED-based light sources with multicolor capability on a single substrate are barely developed.
Variable hue GaN MIS-LEDs which change their color as a function of bias are known from the article "GaN electroluminescent devices: preparation and studies" by G. Jacob et al., Journal of Luminescence, Vol. 17, pp. 263-282, 1978. The wavelength tuning of such LEDs is based on the coexistence of different impurity levels in the energy gap of the host semiconductor and the bias dependence of their occupation with electrons. At low bias, the transitions with the lowest energy occur. However, with increasing bias, the emission due to this transition saturates, whereas a transition with a higher energy appears with increasing intensity and begins to dominate the electroluminescence spectrum at even further increased bias. The article cited above gives the example of a GaN MIS-LED which mixes yellow and blue light with an increasing share of blue light as a function of increasing bias, both colors being generated in the same active region.
A variable hue LED which resembles the preceding example was previously disclosed in the article "Variable Hue GaP Diodes" by W. Rosenzweig et al., Solid-State Electronics, Vol. 14, pp. 655-660, 1970. In this case, GaP is the host material and nitrogen and ZnO are used as dopants which generate different impurity states giving rise to electroluminescence at two wavelengths, red and green, the intensity of both colors being interrelated depending on the bias.
Another concept of integrating different LED-based light sources with different colors on a single substrate is the vertical integration of different active regions, each contributing to a particular one of a plurality of different emission lines. One example in accordance with this approach is given in the article "A Multi-Color GaP LED Flat Panel Display Device" by T. Niina, 1981 DID Int. Symp. Digest Technical Papers 12, pp. 140-141, 1981. The device disclosed consists of a stack of GaP layers, alternatively doped n-, p-, p-, and n-type, thus forming two p-n-junctions on top of each other which are electrically isolated from each other. The active region in one p-n-junction is doped such that impurity induced indirect band-to-band transitions result in the radiation of green light (see above). The other p-n-junction contains ZnO impurities enabling the generation of red light with photon energies below the energy gap of GaP due to transitions to impurity levels within the bandgap. In order to bias both p-n-junctions independently, three electrodes are necessary whereby complicated processing steps are required for their fabrication, namely the etching of isolation trenches for the electrical isolation of the electrodes and the local overcompensation of the top n-GaP layer with acceptors for enabling electrical contact to the p-GaP layers. The possibility of independently biasing each p-n-junction allows for the generation of any intermediate color between red and green, whereby for an observer, the light seems to come from a single light source. According to the above-mentioned reference, single elements of such 2-color LEDs have been made and used as picture elements of large flat panel displays for TV applications.
Up to now, these concepts have not been extended to provide a multitude of multicolor LED-based light sources on a single substrate. In particular, the question of how to provide a multitude of LED-based light sources with predefined colors between blue and red, with predefined shape, and predefined position on a single substrate has not been tackled.